Net-Zero Energy Homes:The Basics of
What You Need to KnowBuilding Science Corporation
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Building Science CorporationKohta Ueno, Senior Associate
The 2010 HVACR & Plumbing Instructor Workshop:Advancing Green Mechanical Concepts
March 24-26, 2010
Background
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Background
Definitions
• NZE: A building that produces as much energy in a typical year as it consumes.
– Consumes grid power when it needs it
– Feed power to grid when it has extra
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– Feed power to grid when it has extra
• ALL energy considered
– Electric is not special.
• NOT Zero Carbon, or Zero GHG
• NOT off-grid
– Much more difficult
Why Buildings?
• Building Sector is largest energy consumer and GHG emitter
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NZE Design Targets
• Produce as much as we consume
• Production is usually MUCH more expensive than reducing waste (efficiency/conservation)
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(efficiency/conservation)
– Hence the energy demanded by building should always be reduced, reduced, reduced before adding production
– Check cost of reducing demand vs cost if supplying energy
Capital Investment vs. Operating Cost
1,500
2,000
2,500T
otal
Ann
ual C
osts
($/
year
)
mortgage
utility bills
cash flow
1
2
3
4
1,500
2,000
2,500T
otal
Ann
ual C
osts
($/
year
)
mortgage
utility bills
cash flow
1
2
3
4
1,500
2,000
2,500T
otal
Ann
ual C
osts
($/
year
)
mortgage
utility bills
cash flow
1
2
3
4
Minimum
Cost Point
“Least Cost”
Curve
Neutral Cost Point
All
En
erg
y R
ela
ted
Co
sts,
$
Lower cost
Higher Energy costs
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0
500
1,000
0% 100%Energy Savings (%)
Tot
al A
nnua
l Cos
ts (
$/ye
ar)
2
0
500
1,000
0% 100%Energy Savings (%)
Tot
al A
nnua
l Cos
ts (
$/ye
ar)
2
0
500
1,000
0% 100%Energy Savings (%)
Tot
al A
nnua
l Cos
ts (
$/ye
ar)
2
Incremental, Energy
Related Mortgage
Costs
Source Energy Reduction
All
En
erg
y R
ela
ted
Co
sts,
$
Underlying Source: Dr Ren Anderson, NREL
Lower cost
conservation
Takeaway Lessons
• Conservation measures first! Good design/orientation, good enclosure (shell), good mechanicals
• Then start adding renewable energy
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• Then start adding renewable energy
• Insulation has diminishing returns
• Renewables can be more cost-effective than insulation after a point!
• Net zero energy: good & noble target, but out beyond “neutral cost”
Renewables
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Renewables
Energy Supply
• Renewable energy (RE) or cleaner energy (CE)
• Net Zero currently demands site
production
– This eliminates some good economical RE
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– This eliminates some good economical RE
• Common choices
– Photovoltaic: Electricity
– Solar thermal Warm / Hotwater
– Combined heat and power
– Wind electricity
• PVphotovoltaic
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Energy Supply (RE)
• PV
– Straightforward installation, easy to predict output
– Expensive but electricity is very useful and
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– Expensive but electricity is very useful and excess can easily be sent to the grid (grid=battery)
– Rated by peak output under standard solar conditions (“peak Watt” or W
p)
– Costs now $8/Wp
(before subsidy) installed
• Solar Thermal“hot water”
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Solar thermal• Intermittent source of hot water
• Well developed
• Requires big storage tanks in most application
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application
• Freezing, over heating, glycol thickening failures, and low temperature efficiency are issues
• Not the most economically-viable choice ($6-10 K): but if going to net zero…
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Combined heat and power
•Aka “CHP”
•Efficient use of fuel to produce heat & electricity
within the building complex (e.g. hospital) or home
•Remember: grid ~30% efficient; waste heat =
cooling towers, river water
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cooling towers, river water
•Much lower GHG emissions
•Supplies on demand
•Ratio of electricity to
heat is fixed
•Effectiveness varies on case-by-case basis
Mechanical
Systems
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Systems
Mechanical Systems
Energy consuming functions
• Heating
• Cooling
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• Cooling
• Domestic Hot Water
• Ventilation & Filtration
Fundamental problem: small loads!
Furnaces• Condensing gas
furnaces: 90%+ AFUE—mature technology
• Sealed
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• Sealed combustion
• ECM motors (“variable speed”) reduces fan electrical energy
Ground-source heat pumps• Uses constant ground
temperature to provide heating & cooling
• Fluid pumped through underground tubes; heat extracted or rejected
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underground tubes; heat extracted or rejected
• One of the highest efficiency space conditioning systems (measured ~3.5 COP)
• But….
Ground-source heat pumps• Installed cost of system very high (drilling
ground loops)
• Nameplate efficiency < actual efficiency (previous example: 5 COP rated number)
• Pumping energy
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• Pumping energy
• Systems with problems—difficult to diagnose, expensive to fix
• Can still suffer from normal ductwork-based system problems
• For small loads, is it worthwhile?
• 80% & 90%+ options
• Condensing boilers (90%+)—needs
Heating: Hydronic Systems
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(90%+)—needs some thinking/design
• Outdoor reset controls for 90%+
• Can’t add cooling
Radiators/Radiant Floors
Radiant floor heat
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Baseboard radiators
Radiant floor heat
Mini Splits
Mini-split short ducted system
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Mini-split non-ducted head
Mini-split outdoor unit
•Both heating & cooling
•Multi-splits (single outdoor unit)
•Systems with SEER26 and HSPF=11 available
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MiniMiniMiniMiniMiniMiniMiniMini--------Splits Heating/Cooling in Cold ClimateSplits Heating/Cooling in Cold ClimateSplits Heating/Cooling in Cold ClimateSplits Heating/Cooling in Cold ClimateSplits Heating/Cooling in Cold ClimateSplits Heating/Cooling in Cold ClimateSplits Heating/Cooling in Cold ClimateSplits Heating/Cooling in Cold Climate
••1818 sf house, solar1818 sf house, solar--oriented, superinsulated (12“ spray foam oriented, superinsulated (12“ spray foam walls, Rwalls, R--80 roof), triple glazed windows, very airtight80 roof), triple glazed windows, very airtight
••Central Massachusetts locationCentral Massachusetts location
••Net zero performanceNet zero performance
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••Provides for both heating & cooling; 11,000 BTU heating loadProvides for both heating & cooling; 11,000 BTU heating load
••Installed costs in the 1,818 square foot “Farmhouse” was $6,850Installed costs in the 1,818 square foot “Farmhouse” was $6,850
••Two 9,000 BTU heads upstairs, One 12,000 BTU head downstairsTwo 9,000 BTU heads upstairs, One 12,000 BTU head downstairs
••Electric heater back up, no heat production below zero degrees Electric heater back up, no heat production below zero degrees outsideoutside
Ventilation
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Ventilation
Need for Ventilation
• Greater airtightness for energy reasons, in net zero houses
• Also improves sound, odor, pest control
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• Also improves sound, odor, pest control
• But people still stink! (+ activities)
• Controlled mechanical ventilation
– Point source control (exhaust fans)
– General dilution ventilation
Air Change Driving Forces
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Wind Effect Stack Effect Combustion and Ventilation
Indoor Air Quality
• Pollutant production
• Pollutant removal
• Dynamic Balance= pollutant level
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• Dynamic Balance= pollutant level
– Not a IAQ problem if it is not in the air
• Solutions
– Reduce pollutant production
– Increase pollutant removal
Ventilation
• Given sensible source control, constant ventilation can dilute pollutants to a low level
– Ventilation rates are mostly about odor and humidity, not oxygen
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humidity, not oxygen
– 7.5 cfm/person + 0.01 cfm / sq ft
– Commercial and highrise 15 cfm/person (!)
• Mixing is necessary or separate supply to each room to achieve best IAQ
Types of Controlled Ventilation Systems
• Exhaust Ventilation
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• Exhaust Ventilation• Supply Ventilation• Balanced Ventilation
Exhaust Only: Depressurize
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• Lowest cost installed system (typical), but problems associated:
• Carbon monoxide alarms
• Lack of filtration
Exhaust pros and cons
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• Lack of filtration• Dust marking on light carpets
• Dirt/grit particles settling on horizontal surfaces
• Lack of distribution• Moisture accumulation and odor buildup in rooms
remote from exhaust fan
• Objections to fan noise
Supply Only: Pressurized
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Not common, although most commercial buildings have
more supply than exhaust
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Plus “fan cycling” controller (runs air handler periodically + motorized damper to prevent overventilation)
Central fan-integrated ventilation
• “Smart” controller (accounts for previous runtime)
• Set minimum runtime (e.g., 20 min/hour)
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• Set minimum runtime (e.g., 20 min/hour)
• Provides distribution of ventilation air throughout house
Balanced Supply and Exhaust
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Common for commercial solutions
Residential often combine with HRV/ERV
Balanced Ventilation (with Heat Recovery)
• HRV/ERV
• Point exhaust
• Fully ducted(need not be)
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(need not be)
Heat Recovery Ventilation
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Efficient Equipment
• HRV/ERV always
– choose better than 1 CFM/Watt (current high end ~2 CFM/Watt)
– Choose > 60% efficient
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– Choose > 60% efficient
– Right size ventilation!—overventilation can defeat the benefits of adding heat recovery!
Humidity
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Humidity
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Recommended Range of Relative Humidity• 25 percent during winter• 60 percent during summer
Supplemental Humidity Control• Good energy efficient design reduces
sensible cooling loads—insulation, good windows, airtightness
• Latent load remains the same!
• Thermostat (temperature control) → humidity is not controlled
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• Thermostat (temperature control) → humidity is not controlled
• Need supplemental dehumidification in hot-humid and mixed-humid climates (high performance houses)
• Demonstrated in 20 research housesInformation Sheet 620: Supplemental Humidity Control
RR-0505: Residential Dehumidification Systems Research for Hot-
Humid Climates
Dehumidification
� High efficiency supplemental dehumidification options (standalone ducted boxes)
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� E.g., draw from main space, dehumidified air to supply duct� Humidistat control in main
space (near thermostat)
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Combustion
Safety
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Safety
Combustion Safety
• Backdrafting risk in tighter houses
• Combustion air should be drawn from outside (“sealed combustion”)
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Case Study:
Westford
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Westford
Case Study House
(Westford Habitat for Humanity)
• Based on recently-built house
• Super-insulated enclosure
• Very airtight (1.5 ACH 50)
• Best-in-class mechanical systems
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• Best-in-class mechanical systems
• Energy Star appliances
• Compact fluorescent lighting
• No renewable energy added: not NZE (PVs or solar DHW)
Westford House: 1.5 Story Single Family Home with Conditioned Basement (2200 ft2 total)
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Westford House Under Construction
� Enclosure Details
• R-66 roof insulation
• R-45 walls
• R-26 basement walls
• R-10 basement
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• R-10 basement slab
• Low e double glazed windows
• 1.5 air changes per hour at 50 Pascals (“ACH50”)
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� Mechanical Details� 96% AFUE Gas
Furnace, ECM motor
� 0.82 EF
Instantaneous Water
Heater
� Fantech Energy
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� Fantech Energy
Recovery Ventilator
(ERV)
� MEL reduction 10%
� 19.3 MBH Heating
� 13.5 MBH Cooling
54
Instantaneous Water Heater
Fantech ERV
100
150
200
250
mill
ion
Btu
/yea
r (s
ourc
e en
ergy
)
100
150
200
250
mill
ion
Btu
/yea
r (s
ourc
e en
ergy
)
100
150
200
250
mill
ion
Btu
/yea
r (s
ourc
e en
ergy
)
100
150
200
250
mill
ion
Btu
/yea
r (s
ourc
e en
ergy
)
53%
Enclosure improvements
Mechanical improvements
Appliance, lights
Parametric Analysis
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0
50
Bench
mar
kAs-
built
base
Air Sea
lR-4
6 W
allR-6
6 Roo
fR-2
6 Bsm
tTr
iple
glaz
ed95
% A
FUE
16 S
EER
HRVDHW
0.8
2 EF
CFLs
ES App
lianc
esSm
art C
ontro
ls
mill
ion
Btu
/yea
r (s
ourc
e en
ergy
)
Heating Cooling Hot Water Lighting Other Reduction 50%
0
50
100
Bench
mar
kAs-
built
base
Air Sea
lR-4
6 W
allR-6
6 Roo
fR-2
6 Bsm
tTr
iple
glaz
ed95
% A
FUE
16 S
EER
HRVDHW
0.8
2 EF
CFLs
ES App
lianc
esSm
art C
ontro
ls
mill
ion
Btu
/yea
r (s
ourc
e en
ergy
)
Heating Cooling Hot Water Lighting Other Reduction 50%
0
50
100
Bench
mar
kAs-
built
base
Air Sea
lR-4
6 W
allR-6
6 Roo
fR-2
6 Bsm
tTr
iple
glaz
ed95
% A
FUE
16 S
EER
HRVDHW
0.8
2 EF
CFLs
ES App
lianc
esSm
art C
ontro
ls
mill
ion
Btu
/yea
r (s
ourc
e en
ergy
)
Heating Cooling Hot Water Lighting Other Reduction 50%
0
50
100
Bench
mar
kAs-
built
base
Air Sea
lR-4
6 W
allR-6
6 Roo
fR-2
6 Bsm
tTr
iple
glaz
ed95
% A
FUE
16 S
EER
HRVDHW
0.8
2 EF
CFLs
ES App
lianc
esSm
art C
ontro
ls
mill
ion
Btu
/yea
r (s
ourc
e en
ergy
)
Heating Cooling Hot Water Lighting Other Reduction 50%
53%
Final Points
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Final Points
What about LEED?• A green points-based rating system:
energy is only one component
• Some serious disappointments in actual energy performance
• USGBC now
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• USGBC nowrequires usage data
• How is the house/building operated?
• ASHRAE 90.1 problems?
Source: BSI-007: Prioritizing Green—It's the Energy Stupid*
Questions/
Comments
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Comments
To Take it Further…
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Mitsubishi SEZ Ducted Indoor unitsMitsubishi SEZ Ducted Indoor unitsMitsubishi SEZ Ducted Indoor unitsMitsubishi SEZ Ducted Indoor unitsMitsubishi SEZ Ducted Indoor unitsMitsubishi SEZ Ducted Indoor unitsMitsubishi SEZ Ducted Indoor unitsMitsubishi SEZ Ducted Indoor units
••Provides for both heating and cooling, 17,000 BTU peak heating loadProvides for both heating and cooling, 17,000 BTU peak heating load
••Installed costs in the 4 BR 2,612 square foot “Carlisle” model was $7,600Installed costs in the 4 BR 2,612 square foot “Carlisle” model was $7,600
••One 15,000 BTU heads upstairs, One 18,000 BTU head downstairsOne 15,000 BTU heads upstairs, One 18,000 BTU head downstairs
••20,000 BTU gas fireplace as back up heating system20,000 BTU gas fireplace as back up heating system
2.5
3.0
3.5
4.0
4.5
5.0C
OP
@ 7
0 F
Inte
rior
Heat Pump Behavior
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0.0
0.5
1.0
1.5
2.0
2.5
-5 5 15 25 35 45 55 65
T out (F)
CO
P @
70
F In
teri
or
Threshold COP 16 SEER w Defrost Lo
16 SEER w Defrost Lo Boston Design T
